The invention relates to a method and apparatus for measuring the spacing between a pair of surfaces, at least one of which is of electrically conductive and/or ferromagnetic material. The invention contemplates utilization of magnetic fields for making distance determinations, and is particularly adaptable for situations where one surface wears with respect to another during use.
The primary applicability of the invention is to machinery in which it is important that a particular clearance be maintained between two metallic surfaces that are moving relative to each other. For instance, in conical plug valves or feeders which are used in the cellulose pulp industry (for example, in connection with rotating pocket feeders for high and/or low pressure system in the continuous digestion of wood chips to cellulosic pulp, such as those sold by Kamyr A.B of Karlstad, Sweden, or Kamyr, Inc. of Glens Falls, N.Y.), a pocketed rotating plug is generally conical in shape and fits into a corresponding conical shaped opening in the stationary housing. Such feeders are exposed to pressures in the range of about 1-15 bars, and it is necessary that the clearance between the rotor and the housing be kept at a minimum in order to avoid excessive leakage of cooking liquor, while at the same time being small enough so that metallic contact between the relatively moving rotor and housing does not occur. By practicing the present invention and by utilizing a control system, the rotor plug of such a feeder can be automatically adjusted in the axial direction in order to obtain the desired results, and significant monetary and technical advantages.
The invention is also applicable to other industries, for instance, where there is a need for measuring the clearance in plain bearings, and to other aspects of the pulp industry, as when measuring the gap in various grinding machines, and the like. For instance, in the pulp and paper industry refiners (also known as defibrators) are used to grind raw cellulosic material, such as chips, into pulp, normally utilizing a stationary grinding surface and a rotating grinding surface, which surfaces may be either of conical or flat configuration. The raw material to be ground passes between the opposing surfaces. In such refiners, it is necessary that the gap between the grinding surfaces be kept at a desired distance (which can be as little as a fraction of a millimeter), and it is also necessary that this clearance be accurately measured and adjusted during operation, taking into account any wear of the grinding surfaces. The surfaces are in most cases rough, having grinding portions disposed in a pattern, and of course the spacing between the surfaces must be controlled so that the grinding portions do not touch each other. It is desirable to provide for automatic control of the grinding surfaces so that a desired optimum gap is kept between them.
There have been a number of previous proposals for utilizing magnetic reluctance between a transducer and a closely located ferromagnetic object for distance measurement, such as shown in U.S. Pat. No. 4,387,339. In such devices, when the magnetic field is produced by direct current, the accuracy of the measurement procedure is often disturbed by temperature variations (since they influence the permeability of the ferromagnetic material). When the magnetic field is produced by an alternating current in such prior proposals, the distance determinations can also be disturbed by possible variations of the resistivity of the ferromagnetic material, or the medium located between the transducer and the ferromagnetic object. Other prior art procedures are based on the utilization of a transducer which generates an alternating current magnetic field which induces eddy currents in an electrically conducting object. The eddy current varies with the distance to the transducer. These measurement techniques are also not necessarily precise since they can be disturbed by variations in the resistivity of the object, and the medium between the object and the transducer.
According to the present invention, a method of measuring the distance between a measuring transducer and an opposing surface is provided which eliminates essentially all of the drawbacks associated with previously known techniques like those discussed above. In the practice of the method, the opposing surface is of an electrically conductive and/or ferromagnetic material, and a measuring transducer is utilized. The measuring transducer includes a core surrounded by coils and establishes a magnetic field, with a gap between the measuring transducer and the opposing surface, which gap comprises part of a magnetic circuit between the measuring transducer and the opposing surface. Further, a reference transducer is utilized similar to the measuring transducer, with a constant gap between the reference surface and a surface substantially identical to opposing surface. The method comprises the steps of: (a) supplying alternating current of known amplitude and voltage to the coils of the transducers to produce magnetic fields with each of the transducers; and (b) determining a change in distance between the measuring transducer and the opposing surface by sensing a difference in electromagnetic properties associated with the measuring transducer and/or the reference transducer.
The invention also relates to apparatus for determining the spacing between a measuring instrument and an opposing surface. The apparatus typically comprises the following components: a measuring transducer having a core surrounded by coils and for establishing a magnetic field; means for mounting the measuring transducer with respect to the opposing surface so that there is a gap between them, the gap comprising part of a magnetic circuit between the measuring transducer and the opposing surface; a reference transducer similar to the measuring transducer; a reference body having a reference surface, the reference surface of substantially the same material as the material of the opposing surface; means for mounting the reference transducer with respect to the reference surface so that there is a constant gap therebetween, the gap comprising part of the magnetic circuit of the reference transducer; means for supplying electrical current to the transducer; and means for determining a change in spacing between the measuring transducer and the opposing surface by sensing a difference in electromagnetic properties associated with the measuring transducer and/or the reference transducer.
Apparatus according to the invention is particularly useful for determining the wear of a body in which one or more measuring transducers are located. According to this aspect of the invention there are provided the following components: First and second substantially identical measuring transducers, each comprising a core surrounded by coils connected up to a source of electrical current; said body having an outer surface; an opposing surface of electrically conductive and/or ferromagnetic material, the opposing surface generally parallel to the outer surface with a gap therebetween; and means for mounting the first and second transducers in the body so that they are spaced from each other within the body in a dimension parallel to the outer surface, and so that they are spaced from the outer surface different distances thereby having a different gap distance between each and the opposing surface, so that wear of the body can be detected.
The invention may be readily utilized in a number of different industries, but has particular advantages in the cellulose pulping industry for use with refiners, high pressure feeders, and the like. The invention utilizes standard electronic measuring instruments and electronic parts so that it easy and relatively inexpense to construct, and the invention may be utilized in new equipment, or may be retrofit to existing equipment.
It is a primary object of the present invention to provide a simple yet effective method and apparatus for determining the spacing between opposing surfaces utilizing electromagnetic techniques. This and other objects of the invention will become clear from an inspection of the detailed description of the drawings, and from the appended claims.